Liquid crystal is an organic compound between solid state and liquid state, and the molecules thereof are orderly arranged. When liquid crystal is heated, it is a transparent liquid; when liquid crystal is cooled, it becomes a cloudy crystalline solid. Owing to the abovementioned characteristic, such a compound is called liquid crystal. The principle of liquid crystal display (LCD) is: liquid crystal is encapsulated in a glass casing, and an electrical filed is applied to the liquid crystal to control its transparency and light permeability. Thus, whether a pixel of a LCD panel is lighting or not can be controlled.
The common LCD can be classified into: TN-LCD (Twisted Nematic LCD), STN-LCD (Super Twisted Nematic LCD), DSTN-LCD (Dual scan Super Twisted Nematic LCD), and TFT-LCD (Thin Film Transistor LCD). TN-LCD, STN-LCD, and DSTN-LCD have similar working principles, and the difference thereof is just in the molecular twist angle. The molecular twist angle of STN-LCD is greater than that of TN-LCD and reaches 180 degrees or even 270 degrees. STN-LCD can be applied to electronic products having a lager display panel, such as electronic dictionaries, entertainment electronic products, personal digital assistants (PDA), mobile phones, and lower grade notebook computers.
Refer to FIG. 1 a diagram schematically showing the electrodes of a general STN-LCD panel. The X electrode group consisting of electrodes X1˜Xn vertically crosses the Y electrode group consisting of electrodes Y1˜Yn to form matrix-like intersections, and each intersection represents a pixel of the panel. The common drive circuit one-by-one scans the electrodes arranged in Y direction, and the scanning speed must be faster than the photogene of human eyes lest the picture appear flickering. According to the data coming from the LCD control circuit, the segment drive circuit sends different voltages to the electrodes arranged in the X direction, and whether a pixel is lighting depends on the potential difference of the electrodes intersected at the pixel.
The common signal respectively has a maximum voltage and a minimum voltage in the positive frame interval and the negative frame interval, and the segment circuit sends out the voltage levels of the display data to determine whether to turn on the pixel.
To enable that LCD can present the effect of gray levels, sophisticated signals are used to drive liquid crystal molecules, and different grades of gray levels will thus appear under the photogene effect of human eyes. The common LCD gray-level technologies include: the FRM (Frame Rate Modulation) mode and the PWM (Pulse Width Modulation) mode.
In the FRM mode, the gray level depends on the number of the turn-on frames among N frames per second. For example, suppose there are N frames per second in a monochromatic display; if a pixel is intended to be full white, the pixel should be turned on N times per second. The principle of controlling the gray level via the FRM mode is to control the turn-on number per second of a pixel to determine the ratio of the turn-on frames to N frames per second; thus, the gray level is determined by the turn-on ratio. Refer to FIG. 2 a diagram showing the relation between the gray-level effects (G0˜G3) and the number of the turn-on frames per second when there are three frames (FR0˜FR2) per second (4FRMA—4 gray-level FRM mode). If the pixel is turned on 3 times, the pixel will be full white (of G3 gray level); if the pixel is turned on 0 time, the pixel will be black (of G0 gray level).
Such a mode only varies the turn-on number of frames; therefore, only the LCD control circuit needs changing, and it is unnecessary to change the LCD drive circuit. The key point of the FRM mode is the frame frequency, i.e. the number of the frames per second; the displaying speed of the frames must be faster than the photogene of human eyes and is usually within 42˜140 Hz; otherwise, flickering phenomenon will appear. In the FRM mode, to achieve an effective gray-level effect, the displaying time of each pixel should be increased to accumulate enough light for photogene, and the frequency overlap with the background light should be prevented also.
In the PWM mode, the gray level is controlled via adjusting the length of the turn-on period within each frame interval. For example, in the PWM mode, if the frame frequency is N frames per second, a pixel will be turned on N times per second; however, only a portion of a frame interval will be turned on, and the gray level depends on the proportion of the turn-on portion within a frame interval.
Refer to FIG. 3 a diagram showing the 3PWM in SEG mode with the horizontal synchronous signal Hsync and the vertical synchronous signal Vsync of the same frequency. In 3PWM mode, a frame interval is divided into two sub-sections; thus, according to the combination of the turn-on and non-turn-on states of those two sub-sections within a frame interval, a pixel may have three gray-level states: non-turn-on, half turn-on, and full turn-on, and FIG. 3 shows that a frame interval is half turned on. The PWM mode needs to adjust the output timing of the drive circuit; therefore, both the original LCD control circuit and the original LCD drive circuit need changing, i.e. the original segment drive circuit has to be replaced with a specially designed segment drive circuit; thus, the complexity and cost of the circuit will be considerably increased.